Steel fiber for reinforcement of high-performance concrete

ABSTRACT

A steel fiber ( 10 ) for reinforcement of high-performance concrete or mortar has a length ranging from 3 mm to 30 mm, a thickness ranging from 0.08 mm to 0.30 mm, and a tensile strength greater than 2000 MPa. The steel fiber is provided with anchorages ( 12,24 ) the dimension of which in a direction perpendicular to the longitudinal axis of the steel fiber is maximum 50% of the thickness. These anchorages provide an effective staying in the high-performance concrete without influencing the mixability of the steel fibers in a negative way.

FIELD OF THE INVENTION

The invention relates to a straight steel fiber for reinforcement ofhigh-performance concrete or mortar.

BACKGROUND OF THE INVENTION.

It is known in the art to reinforce high-performance concretes by meansof steel fibers.

BE-A3-1005815 (N.V. BEKAERT S.A.) teaches that for conventionalconcretes with a compressive strength ranging from 30 MPa to 50 MPa, itmakes no sense to increase the tensile strength of a steel fiber above1300 MPa since an increase in tensile strength does not add any increasein flexural strength to the reinforced concrete. BE 1005815 furtherteaches, however, that for concretes with an increased compressivestrength, the tensile strength of the steel fibers should increaseproportionally.

WO-A1-95/01316 (BOUYGUES) adapts the average length of metal fibers tothe maximum size of granular elements which are present inhigh-performance concrete so that metal fibers act as conventionalrebars in high-performance concrete. The volume percentage of metalfibers in high-performance concrete is relatively high and ranges

DE-A1-33 47 675 (LAMPRECHT Gerd) relates to an artificial stone ofcement or gypsum reinforced by means of thin fibers made of ahigh-alloyed steel. The high-alloyed steel fibers are provided withroughnesses on their surface in order to increase the adhesion in thecement and the gypsum. The fibers have a diameter ranging from 0.05 mmto 0.15 mm and the depth of the roughnesses is limited to 30% of thediameter of the fibers.

SUMMARY OF THE INVENTION

It is an object of the present invention to further optimize thegeometry and the tensile strength of steel fibers to high-performanceconcrete.

It is also an object of the present invention to reduce mixing problemswhen reinforcing high-performance concrete with high volume percentagesof steel fibers.

It is another object of the present invention to improve the anchorageof steel fibers in the reinforcement of high-performance concrete.

According to one aspect of the present invention, there is provided astraight steel fiber for reinforcement of high-performance concrete ormortar. The steel fiber has a length ranging from 3 mm to 30 mm, athickness ranging from 0.08 mm to 0.30 mm and a tensile strength greaterthan 2000 MPa, e.g. greater than 2500 MPa, or greater than 3000 MPa. Thesteel fiber is provided with anchorages the dimension of which in adirection perpendicular to the longitudinal axis of the steel fiber ismaximum 50%, e.g. maximum 25%, e.g. maximum 15%, of the thickness.

The terms ‘high-performance concrete or mortar’ refer to concrete ormortar the compression strength of which is higher than 75 MPa (1 MPa=1Mega-Pascal=1 Newton/mm² ), e.g. higher than 200 MPa. The compressionstrength is the strength as measured by ASTM-Standard N° C39-80 on acube of concrete of 150 mm edge, where the cube is pressed between twoparallel surfaces until rupture.

The term ‘thickness’ of a steel fiber refers to the smallestcross-sectional dimension of a straight steel fiber without theanchorages.

The term ‘anchorage’ refers to any deviation from a straight steel fiberwith a uniform transversal cross-section where the deviation helps toimprove the anchorage or staying of the steel fiber in the concrete.

Within the context of the present invention, the terms ‘straight steelfiber’ excludes normal bendings but does not exclude small bendings,i.e. bendings with a high radius of curvature, in the steel fiber whichare a result of the steel wire having been wound on a spool before thefinal drawing and/or cutting. Steel fibers with only such small bendingswhich are the result of the previous winding of the steel wire, arestill considered as ‘straight steel fibers’.

The advantage of the present invention may be explained as follows.Concretes have a so-called interfacial zone between the cement paste andaggregates added to the concrete. This interfacial zone can be studiedby means of a scanning electronic microscope (SEM). It has been observedthat due to an increased presence of water in the neighbourhood of theaggregates, cement hydration is accelerated in the interfacial zone,resulting in the presence of calcium hydroxide intermixed withcalcium-silica-hydrates and ettringite in the interfacial zone. Theconsequence is an interfacial zone with a relatively high degree ofporosity. This interfacial zone forms the weakest link of the concreteand determines to a large extent its strength which tends to be smallerthan the strength of its cement paste. The thickness of the interfacialzone ranges from about 50 μm (micrometer) to about 100 μm around theaggregates. A similar interfacial zone has been observed around steelfibers added to the concrete.

In comparison with conventional concretes, high-performance concretesare characterized by:

(a) a relatively low water/cement ratio (smaller than 0.45);

(b) the addition of superplasticizers which much increase theworkability of concrete in spite of the low water/cement ratio;

(c) the addition of mineral additives such as silica fumes, fly ashes,blast furnace slag, pulverized fuel, micro-fillers and/or pozzolansand/or the addition of chemical additives such as water glass andtensides.

The additives mentioned under (c) result in an increased bond betweenaggregates and cement and result in an interfacial zone the thickness ofwhich is substantially decreased, if not disappeared. Indeed silicafumes, for example, transform the calcium hydroxides of the interfacialzone into calcium-silica-hydrates.

In order to have an effective anchorage or staying in conventionalconcretes, steel fibers must have anchorages with dimensions that are afew times the thickness of the interfacial zone, i.e. a few times 50μm-100 μm. Anchorages with smaller dimensions will not work to the samedegree, since they would not bridge adequately the interfacial zone. Incontradiction with the interfacial zone of conventional concrete, theinterfacial zone of high-performance concretes is either not so weak ornot so thick or even not existent. The result is that steel fibersprovided with anchorages of a small dimension work effectively.

A supplementary advantage of the smaller dimensions of the anchorage isthat the mixing problem of steel fibers in the concrete is reduced sincethere are no substantial bendings any more.

Another advantage is that, due to the improved anchorage, the volume ofsteel fibers needed for a required performance of the concrete, may bereduced, which also reduces considerably the degree of mixing problems.This is very important since the volume percentage of steel fibers inhigh-performance concrete is substantially higher (normally 1.0% to4.0%) than in conventional concretes (normally 0.40% to 1.0%), and thehigher this volume percentage the greater the risk for mixing problems.

Within the context of the present invention the anchorages are notlimited to a particular form or way of manufacturing. The anchorages maytake the form of bendings or waves on condition that their dimension ina direction perpendicular to the longitudinal axis of the steel fiber islimited in size. The anchorages may also take the form ofmicro-roughenings, e.g. obtained by means of a controlled oxidation orby means of a controlled etching operation.

In a first preferable embodiment of the invention the anchorages areindentations which are distributed along the length of a straight steelfiber. The depth of these indentations ranges from 5% to 25% of thethickness of the steel fiber without indentations. For example, thedepth of these indentations ranges from 0.01 mm to 0.05 mm. Theindentations may be provided at regular distances along the length ofthe steel fiber.

In a second preferable embodiment of the invention the steel fiber isprovided with flattenings at both ends of the steel fiber. The thicknessof the flattened ends may range from 50% to 85% of the thickness of thenon-flattened steel fiber. Such a steel fiber has preferably anelongation at fracture which is greater than 4%.

In order to provide the required tensile strength, a steel fiberaccording to the present invention preferably has a carbon content above0.40%, e.g. above 0.82%, or above 0.96%.

According to a second aspect of the present invention, there is provideda method for improving the mixability of steel fibers inhigh-performance concrete, said concrete having a compressive strengthgreater than 75 MPa, said method comprising the steps of:

(a) providing straight steel fibers; said steel fibers having a lengthranging from 3 mm to 30 mm, a thickness ranging from 0.08 mm to 0.30 mm,

(b) providing anchorages in said steel fibers, said anchorages having adimension in a direction perpendicular to the longitudinal axis of thesteel fibers of maximum 50% of the thickness of the steel fibers.

Or viewed from another angle, there is provided a method of adapting theanchorages of a steel fiber to the dimensions of an interfacial in ahigh-performance concrete or mortar. The method comprises the followingsteps:

(a) providing a steel fiber with a length ranging from 3 mm to 30 mm, athickness ranging from 0.08 mm to 0.30 mm, a tensile strength greaterthan 2000 MPa,

(b) providing said steel fiber with anchorages the dimension of which ina direction perpendicular to the longitudinal axis of the steel fiber ismaximum 50% of the thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1(a) gives a global view of a steel fiber provided withindentations along its length;

FIG. 1(b) gives an enlarged view of an indentation;

FIG. 2 schematically illustrates how a steel fiber with indentations canbe manufactured;

FIG. 3(a) gives a side view and FIG. 3(b) gives an upper view of a steelfiber with flattened ends;

FIG. 4 schematically illustrates how a steel fiber with flattened endscan be manufactured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

First Preferable Embodiment

FIG. 1(a) shows a steel fiber 10 which is provided with indentations 12which are regularly distributed along its length. FIG. 1(b) illustratesin more detail an indentation 12. For example, the steel fiber 10 has alength of 13 mm, and—apart from the indentations 12—a roundcross-section with a diameter of 0.20 mm. The size a of an indentation12 in the longitudinal direction is 0.50 mm and the depth b of anindentation 12 is 0.010 mm (=10 μm). The indentations 12 are providedboth at the upper side and at the under side of the steel fiber 10. Thedistance (pitch) between two indentations at the upper or at the underside is about 1.50 mm.

FIG. 2 illustrates how a steel fiber 10 with indentations 12 can bemanufactured. A steel wire 14 is drawn by means of a winding drum 16through a (final) reduction die 18. Having reached its final diameterthe wire 14 is further guided to two wheels 20 which are both providedat their surface with protrusions 21 in order to form the indentations12 in the wire 14. The two wheels 20 give the necessary pulling force toguide the wire 14 from the winding drum 16 to a cutting tool 22 wherethe steel wire 14 is cut into steel fibers 10 of the same lengths.

Second Preferable Embodiment

FIGS. 3(a) and 3(b) illustrate a straight steel fiber 10 with flattenedends 24. The flattened ends 24 provide the anchorage in thehigh-performance concrete. Preferably the steel fiber 10 has no burrssince burrs could provoke concentrations of tensions in the concrete andthese concentrations could lead to initiation of cracks. The transitionin the steel fiber 10 from the round transversal cross-section to theflattened ends 24 should not be abrupt but should be gradually andsmooth. As an example the steel fiber 10 has following dimensions: alength of 13 mm, a diameter of a round cross-section of 0.20 mm, athickness d of the flattened ends 24 of 0.15 mm and a length e of theflattened ends 24—transition zone included—of 1.0 mm.

FIG. 4 illustrates how a steel fiber 10 with flattened ends 24 can bemanufactured by means of two rolls 26 which give flattenings to a steelwire 14 and simultaneously cut the steel wire into separate steelfibers.

Since a steel fiber 10 according to this second embodiment will beanchored in the high-performance concrete only at the ends 24 (and notalong its length as in the first embodiment), it is preferable toincrease the potential of plastic energy in the steel fiber by applyinga suitable thermal treatment in order to increase the elongation atfracture of the steel fiber 10. Such a thermal treatment is known assuch in the art. The thermal treatment can be applied by passing thesteel wire 14 through a high-frequency or mid-frequency induction coilof a length that is adapted to the speed of the steel wire and to heatthe steel wire 14 to about more than 400° C. The steel wire will sufferfrom a certain decrease of its tensile strength (about 10 to 15%) but atthe same time will see its elongation at fracture increase. In this waythe plastic elongation can be increased to more than 5% and even to 6%.

The composition of the steel fiber may vary to a large extent.Conventionally it comprises a minimum carbon content of 0.40% (e.g. atleast 0.80%, e.g. 0.96%), a manganese content ranging from 0.20 to 0.90%and a silicon content ranging from 0.10 to 0.90%. The sulphur andphosphorous contents are each preferably kept below 0.03%. Additionalelements such as chromium (up to 0.2 à 0.4%), boron, cobalt, nickel,vanadium . . . may be added to the composition in order to reduce thedegree of reduction required for obtaining a particular tensilestrength.

The steel fiber can be provided with a coating such as a metalliccoating. For example it can be provided with a copper alloy coating inorder to increase its drawability or it can be provided with a zinc oraluminum alloy coating in order to increase its corrosion resistance.

The steel fiber according to the present invention is not limited toparticular tensile strengths of the steel fiber. For steel fibers of0.20 mm thickness tensile strengths can be obtained ranging frommoderate values of 2000 MPa to higher values of 3500 MPa, 4000 MPa andeven higher. It is preferable, however, to adapt the tensile strength ofthe steel fiber both to the compression strength of the high-performanceconcrete and to the quality of the anchorage in the high-performanceconcrete. The higher the degree of anchorage in the concrete, the moreuseful it is to further increase the tensile strength of the steel fiberitself.

The steel fibers according to the invention may be glued together bymeans of a suitable binder which looses its binding ability when mixingwith the other components of the high-performance concrete. The applyingof such a binder increases the mixability, as has been explained in U.S.Pat. No. 4,224,377. However, in the context of the present invention,this is not strictly necessary.

What is claimed is:
 1. A concrete or mortar having a compressivestrength greater than 75 MPa, comprising: aggregates; cement paste; aninterfacial zone between said cement paste and said aggregates; andsteel fibers said steel fibers comprising a length ranging from 3 mm to30 mm, a thickness ranging from 0.08 mm to 0.30 mm, and anchoragescomprising a dimension in a direction perpendicular to said length ofsaid steel fibers that is a minimum of 0.01 mm and a maximum of 50% ofsaid thickness configured so as to bridge said interfacial zone.
 2. Aconcrete or mortar according to claim 1 wherein said dimension of saidanchorages in a direction perpendicular to said length of said steelfibers is a maximum of 25% of said thickness.
 3. A concrete or mortaraccording to claim 1 wherein said dimension of said anchorages in adirection perpendicular to said length of said steel fibers is a maximumof 15% of said thickness.
 4. A concrete or mortar according to claim 1wherein said anchorages are indentations distributed along the length ofsaid steel fibers.
 5. A concrete or mortar according to claim 4 whereinsaid indentations have a depth dimension in a direction perpendicular tosaid length of said steel fibers that ranges from 0.01 mm to 0.05 mm. 6.A concrete or mortar according to claim 1 wherein said anchorages areflattenings at both ends of said steel fibers.
 7. A concrete or mortaraccording to claim 6 wherein said steel fibers have a total elongationat fracture greater than 4%.
 8. A concrete or mortar according to claim1 wherein said steel fibers have a carbon content greater than 0.40%. 9.A concrete or mortar according to claim 8 wherein said steel fibers havea manganese content ranging from 0.10% to 0.90% and a silicon contentranging from 0.10% to 0.90%.
 10. A method for improving the mixabilityof steel fibers in high-performance concrete with a compressive strengthgreater than 75 MPa, comprising the steps of: providing a concrete ormortar comprising cement paste and aggregates, and an interfacial zonebetween said cement paste and said aggregates; providing straight steelfibers comprising a length ranging from 3 mm to 30 mm, a thicknessranging from 0.08 mm to 0.30 mm, and anchorages, said anchoragescomprising a dimension in a direction perpendicular to said length ofsaid steel fibers of a minimum of 0.01 mm and a maximum of 50% of saidthickness of said steel fibers so as to bridge said interfacial zone;and mixing said steel fibers in said concrete or mortar.
 11. A methodaccording to claim 10, wherein said steel fibers have a tensile strengthof at least 2000 MPa.
 12. A method of adapting the anchorages of steelfibers to a zone thickness dimension of an interfacial zone in ahigh-performance concrete or mortar, said method comprising the stepsof: providing steel fibers comprising a length ranging from 3 mm to 30mm, a thickness ranging from 0.08 mm to 0.30 mm, and a tensile strengthgreater than 2000 MPa; and forming anchorages in said steel fibers witha dimension in a direction perpendicular to said length of said steelfiber of a minimum of 0.01 mm and a maximum of 50% of said thickness, sothat said dimension in a direction perpendicular to said length exceedsthe zone thickness dimension of the interfacial zone.